I'm only running at 4500 RPM on my 85hp

[wildmaninal]

If my rpm gauge is reading correctly I'm only running at 4500 rpm at WOT when I know I should be running about 5000 to 5500.

I have this 1978 85hp on a 15ft tri hull bass boat. I haven't done anything with the prop, I believe it is the original. Been trying to read up on the service manual but I really only see about synchronizing on the carbs. Which I believe the carbs are in sync but I have noticed that it is running a little to low on idle which it has been known to die at idle, haven't paid attention to the rpm at idle though :redface:. It also takes a moment for the boat to get on plane.

I just want to make sure I am going on the right track. Should I just adjust the throttle trunnion nut? Will I gain that extra 1000 RPMs by doing so or no? What rpm should I be at at idle?

I have been using my boat like this and I actually can keep up with my brother's 1975 115 hp on a 75model 17ft bass boat hull. I believe my rpm gauge is accurate though for real. I also have the old style 2 handle remote hooked to this motor this might have something to do with it I don't know.

 

[F_R]

Re: I'm only running at 4500 RPM on my 85hp

The trunnion nut definatly will cause your problem if not adjusted to provide Wide Open Throttle. Simply see if is doing that and adjust it accordingly if is not doing that. There is no reason the dual levers control shouldn't work ok. It is a bit unusual though since the motor came with a single lever control from the factory.

Otherwise, WOT RPM is determined by the power output of the engine and the load that it is pushing. So you have to first determine if the motor is healthy. Good (and equal) compression on all four cylinders? Firing equally on all 4? Both carburetors and ignition system come into play as regards firing equally on all 4.

Only once you know for sure that all the above is correct, should you select a propeller of the correct diameter and pitch that lets the motor turn at the desired RPM range. BTW, there was no "original" propeller. It was the dealer's responsibility to select and install the correct prop to match the boat when he sold it new. Not knowing any more than I do about your boat, my wild guess is that it probably would be good with a 17" pitch prop. Am I close?

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

Well I rebuilt the carbs and did the link and sync myself a while back, obviously it has been running like that ever sense. I had to replace the one coil pack, I'm pretty sure the firing is alright on this motor.

As for the prop I couldn't tell ya what pitch it is. I know that the motor was on a Thunder craft ski boat hull which might be a lighter hull then this bass boat hull but otherwise the Thundercraft is really the same shape hull and same size.

I am just hoping that I don't get to high on the RPMs at idle in exchange for full WOT at 5500 rpm. I know I do need to do a little something with the idle though, which will ultimitely change the throttle maybe for the better.

I'll just mark the throttle cable trunnion nut and at worst I will just half to move it back to where it was. I have replaced the throttle cable in the past.

 

[jay_merrill]

Re: I'm only running at 4500 RPM on my 85hp

Do you know how fast the boat is going at WOT?

 

[70mstang]

Re: I'm only running at 4500 RPM on my 85hp

you said you rebuilt the carburetor right? maybe you should see if you mixed up the low and high jets, i've done that before, very hard to figure out

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

Do you know how fast the boat is going at WOT?

About 45 to 50 MPH. Maybe a hair less, my brother's boat will do about 50 to 55 or so and I was keeping up with him.

70mstang, I don't think I got them jets mixed up.

 

[F_R]

Re: I'm only running at 4500 RPM on my 85hp

Ain't no way you are doing 50 unless you have a mighty steep prop on it. And no wonder your RPM is down. Must be a nice boat too.

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

Ain't no way you are doing 50 unless you have a mighty steep prop on it. And no wonder your RPM is down. Must be a nice boat too.

Well I know my brother had his boat clocked by my uncles boat/GPS in the past and his would do 50mph. Now I can tell ya he had some water in that hull durring 4th of July weekend and that would have slowed him down some I know.

So do you think it is my prop after all FR? My boat is a heavy boat for a 15ft tri hull bass boat. Not the best looking boat but it floats and does the job.

I really need to take a close look at the prop I guess and maybe even take a picture of it. Well I might have a picture already.

I am planning on taking it out on the lake within the next couple of days to do some fishing and I might just tinker with the motor then instead of messing with the motor while on muffs. My brother does have some spare props but he bought them for his 115hp Evinrude 75 model. I don't know if those props will work on mine, they should I guess.

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

Well here is a picture of my boat (before I got it lake worthy) and a picture of my prop.

 

[F_R]

Re: I'm only running at 4500 RPM on my 85hp

That's a stainless steel prop, right? Nice props. Take the prop nut off, and the brass washer behind the nut. The diameter and pitch are marked on the prop hub, behind the brass washer. Then we at least will know what we are dealing with.

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

I believe it is a stainless steel prop.

The numbers I got off of it was 12 3/4 x 21.

 

[Dennisanoka]

Re: I'm only running at 4500 RPM on my 85hp

12 3/4" and 21 pitch

Assuming your engine is running correctly there are two things you can do to improve your rpm. Either drop down to a lower pitch prop and you will gain approx. 200 rpm for ea. size you go down. The next size down is 19 then 17 and so on. The other thing you can check is the ht. of your motor on the transom. If it is too low in the water it provides too much drag. Look at the cavitation plate, it should be approx. even with the bottom of your boat. The draw back of raising it too much is that you will begin to cavitate on high speed turns or pulling a skier or tube.

 

[Dennisanoka]

Re: I'm only running at 4500 RPM on my 85hp

I'm with F_R I would have guessed a 17 size prop on this eng. 21 does seem to be over proped which as you know will cause excessive carbon build up and could spell a short life to your prized engine.
My 90 came with a 19 and I since put on a 17 but I'd even drop to a 15 next time around.

I'd sugggest doing a de-carb on your engine once you get it set up right. Just do a search on de-carbing engine using Seafoam. Very cheap and very effective.

 

[Edmac]

Re: I'm only running at 4500 RPM on my 85hp

I have the same motor on my 16' Bass Boat. My OEM Manual
states that it was supplied with a 13 1/4 x 17 prop which I
run and with 2 people in the boat will turn 4900 RPM and
makes about 40 MPH.
I also have a 13 3/4 x 15 which turns 5200 RPM and 37 MPH.
give or take a little for wind and current.

 

[F_R]

Re: I'm only running at 4500 RPM on my 85hp

wildman, you are running at the low end of the RPM range but getting impressive speed. If you want to increase the RPM a bit to be merciful to the motor, drop down to at 19" prop. You will get improved hole shot, and probably about the same top speed. But if you drop back to an aluminum prop you probably will lose some performance. Trouble is, playing with props gets expensive. Those SST's don't come cheap.

 

[wildmaninal]

Re: I'm only running at 4500 RPM on my 85hp

Thanks for all the help.

Dennisanoka, that cavitation plate is right there even with the bottom of the boat.

I need to get in touch with my brother and see what size props he has up there. I know of one other person that may have a 19" prop if my brother doesn't come through for me. Best thing is with my brother I would get a family discount most likely , he's not a boat parts salesman he just has a couple of extra stainless steel props laying around.

That would explain why it would take forever to plane out. I'm not to familiar with props to be honest.

Thanks again everybody.

 

[wildmaninal]

Re: (update) I'm only running at 4500 RPM on my 85hp

Re: (update) I'm only running at 4500 RPM on my 85hp

I thought I would update this because I have just recently started trying to improve the RPM/WOT.

I got a (beat up) aluminum 17" pitch prop from my uncle and finally tried it out the last holiday weekend. It did improve the RPM at WOT, but when I had a passenger on board the boat would be the same when trying to plane out.

So I put the original 21" pitch prop back on and put a whale tail on the foot of the motor, this helped out on the plaining and all but of course didn't help me in getting 5500 RPM at WOT, it also slowed me down about 5mph to about 35mph at WOT. I finally broke down and did some adjusting on the carbs. I took the boat out for a test run, brought my dad with me (his first time on my boat). Once I got the motor trimmed out it would jump up to 5000 RPM. I think if found a "stainless steele" 17" pitch prop that I would finally get 5500 rpm.